The present invention relates to image display systems. In particular, the present invention relates to miniature image display system usable for helmet-mounted displays and other wearable applications.
In the field of miniature image display systems there are continuing challenges and drive to design smaller, lighter, and more energy efficient systems. These challenges stem from the fact that a miniature image display system should preferably be small enough and light enough to be wearable mounted on a helmet or on eyeglasses. And, preferably, the miniature image display system should be foldable for easy storage and transport. Such systems may be used for wearable computer systems, gaming systems, distance interactions between people or between people and machines, virtual-reality system, and for many other applications.
Typically, desktop computer systems and workplace computing equipment utilize CRT (cathode ray tube) display screens to display images for a user. The CRT displays are heavy, bulky, and not easily miniaturized. For a laptop, a notebook, or a palm computer, flat-panel display is typically used. The flat-panel display may use LCD (liquid crystal displays) technology implemented as passive matrix or active matrix panel. The passive matrix LCD panel consists of a grid of horizontal and vertical wires. Each intersection of the grid constitutes a single pixel, and is controlled by a LCD element. The LCD element either absorbs or reflects light depending upon orientation of the pixel. In fact, using quarter wave plates, flat-panel displays reflect light that is 90 degrees rotated from the incident light. That is, when light polarized in a first direction hits the flat panel and reflects off its pixels, the reflected light is polarized in a second, orthogonal direction.
The flat-panel display typically requires external lighting to allow human eyes to see the images displayed on the display panel. This is because flat-panel displays do not generate their own light. For laptop, notebook, or palm computers, the external lighting is typically positioned at the back of the flat-panel. The backlighting allows the user to see the images from the front of the flat-panel.
The flat-panels are also used for miniature image display systems because of their compactness and energy efficiency compared to the CRT displays. For miniature image display systems, reflective lighting, rather than the backlighting, is preferred. This is because, using the reflective lighting technique, miniature image display systems can be designed having higher energy efficiency compared to the energy efficiency of image display systems designed using the backlighting techniques. Various configurations of miniature display systems using flat-panels displays and reflective lighting technique can be found in U.S. Pat. No. 5,808,800.
To further increase the energy efficiency, use of polarizing beam splitting cube ("PBS cube") has been proposed. However, the PBS cube comes at a price in terms of weight and bulk. Installation of the PBS cube within the miniature image display system introduces additional weight and bulk to the system Typically, the PBS cube occupies majority of the space of the miniature image display system. And, because of the PBS cube, the miniature image display system cannot be folded for easy storage or transport. Furthermore, the energy efficiency gain is limited because each time light enters or exits the PBS cube, the light is attenuated, thus losing energy. Configurations of miniature display systems using the PBS cube can be found in U.S. Pat. No. 5,596,451.
Therefore, there still exists continuing need for more compact, lightweight, energy efficient, and foldable miniature display system usable for helmet mounted or otherwise wearable display applications.